a roller coaster and the ups and downs of temperature in Earth's atmosphere. Because the universe as a whole is a self-contained system, conservation of energy applies to the entire cosmos as well. Energy's shape-shifting abilities make it hard to follow from one form to the next, but a few examples will illustrate how central it is to the workings of our world.

We are most familiar with the energy of motion, called kinetic energy . Everything that moves has kinetic energy . The amount goes up quickly as the speed of an object increases. If you double its speed, it carries four times more kinetic energy . Tripling its speed increases the kinetic energy by a factor of nine, and so on. This simple relationship means that tiny objects can carry a lot of kinetic energy if they travel fast enough. For instance, imagine the difference between trying to catch a pellet thrown to you and one fired from an air gun. In space, a collision with something as small as a fleck of paint can inflict serious damage on a satellite or the windows in the Space Shuttle because objects in orbit can collide at relative speeds of thousands of miles per hour.

kinetic energy routinely swaps back and forth with a less obvious type of energy, called potential energy . That switch occurs in the presence of any force that can move objects. Consider an archer's bow and arrow. The archer stores potential energy in the bow by doing work on it--drawing the cord back to pull the flexible bow into a taut "C" shape. Releasing the cord transfers most of that potential energy to the arrow. The arrow then flies away with enough kinetic energy to reach and pierce its target. When a tennis player hits a ball, the combined kinetic energy of the ball and the racket bends the racket's strings and crushes the ball into an oblong shape. In an instant the potential energy held within those distorted shapes converts back into kinetic energy and propels the ball in the opposite direction with renewed vigor.

The Mars Pathfinder spacecraft used this same type of exchange in a unique way when it landed on Mars in 1997. To absorb the kinetic energy of impact, large balloons inflated around the vessel before it hit the ground. Most of that energy was briefly stored as potential energy within the compressed balloons. Then Pathfinder bounced, hit the ground, and bounced again several times. By the time the kinetic-to-potential energy flip-flop dissipated into other forms (such as friction, dents on the ground, and heat within the balloons), the craft came to an undamaged halt.

The most pervasive form of potential energy on Earth and in the universe is gravitational potential energy . Earth's gravity will accelerate any object toward the planet's center when given a chance. To endow something with potential energy , all you need to do is lift it. If you let go, that potential for motion converts into kinetic energy as the object falls. When the floor gets in the way, the kinetic energy transforms into acoustic energy --thud!--and perhaps enough mechanical energy to crack or shatter the object. If you've ever watched a glass plummet from the edge of your dinner table, you know exactly how this works. In a similar way, engineers at amusement parks design conveyor belts to do work on roller coaster s and their passengers. From high atop the first peak, the gravitational potential energy of cars and riders converts into a dizzying rush of acceleration, rotation, and high-speed motion.

More often than not, something prevents an object from gaining speed continuously as it falls. Resistance from air or water is a good example. The converted potential energy then must reveal itself in some other way, frequently as heat . In the mid-nineteenth century, the English physicist James Joule explored this phenomenon with a clever device. Falling weights powered the motions of rotating paddles, which in turn stirred a jar of water. The potential energy of the weights could not transfer entirely into kinetic energy to drive the paddles because the water retarded those motions. As a result, the water warmed slightly. The same effect should apply to water plunging 160 feet over Niagara Falls , Joule reasoned. He calculated that resistance to freely falling motion in Niagara Falls should heat the water by one-fifth of a degree. (To recognize Joule's work on heat and the energy of moving objects, we now call one unit of energy a "joule.") Joule could not foresee that his Niagara Falls thought experiment would serve as a perfect analogy for gas heat ing up as it falls into a black hole. The cosmic distances and temperatures in such a system--billions of miles and millions of degrees--lead to bursts of energy that we can detect across the universe (page 150).

The conversion of energy from potential to kinetic to heat happens all the time, both locally and everywhere in the cosmos. When you drive your car down a steep hill, your potential energy converts to kinetic energy . Apply the brakes and you prevent a dangerous acceleration by converting much of your car's kinetic energy into warming up the brake pads and the rubber tires. The Space Shuttle 's controlled reentry into Earth's atmosphere is marked by intense heat ing of the specially designed tiles that cover it. The tiles grow as hot as 3,000 degrees Fahrenheit. They swiftly radiate this (continued)